1. Field of the Invention
The present invention relates to methods and apparatus using fluidized bed principles for separating mixtures of solid articles of different densities, and more particularly two such methods and apparatus as are applicable to the gading of agricultural products or the separation of agricultural products from associated waste materials.
2. Background Art
The use of density variation as a means of separating articles is widespread. In agriculture the separation and sorting of products on this basis is accomplished using both wet and dry methods.
Wet methods use a liquid as a medium in which to separate denser articles, which will sink in the given liquid, from lighter ones that will float thereupon. Current density-based agricultural sorting techniques that use liquids employ water and various solutions thereof that include, for example, salt or alcohol.
Because of the use of fluids, these techniques have inherent disadvantages which limit their application with agricultural products. Many of the liquids employed are expensive and present fire and social hazards when used in large quantities. Some agricultural commodities, such as peas or blueberries, need preliminary prewetting in order to remove air bubbles and permit their effective sorting using fluids. Other products, such as peanuts, walnuts, and pecans, cannot generally be processed in any liquid because the absorption of liquid adversely affects the properties of the product.
In instances where sorting requires grading of agricultural products into three or more categories, the use of several liquids in succession or the changing of liquids in a single sorting apparatus is required. The preconditioning of produce prior to wetting or the rinsing of produce subsequent to sorting are also often necessary when liquids are used for this purpose. These operations often deteriorate product quality and the freedom with which the products may be stored thereafter. In addition, the liquids involved frequently become contaminated with foreign materials during the sorting process, affecting their density and requiring periodic changing or filtering of the liquids.
Dry methods of sorting or cleaning of agricultural products are not afflicted by the above-described disadvantages. Some dry methods of sorting employ a form of pneumatic separation based on a combination of differing density and differing aerodynamic properties associated with the components to be separated. In such separation schemes, a gas, such as air, is forced upwardly through a moving bed of the mixture to be separated. This gas flow through the interstices of the particles of the mixture tends to disengage those particles from each other permitting the gas flow to support at least some of the weight thereof. As a result, the bed resembles a liquid of high viscosity and the particles of the mixture are freed to a degree to migrate within the bed under the influence of physical forces that might tend to induce a separation among the constituent components. In this respect, such methods employ fluidized bed principles.
Nevertheless, the separation that occurs when a mixture to be separated is itself fluidized is not one that occurs due exclusively to differing density among the components of the mixture. Instead, the aerodynamic properties of the particles of the mixture also have a substantial impact upon the rate and quality of separation that results, as the upward flow of gas through the bed of the mixture will tend to draw with it the less compact particles of the mixture, regardless of their density. Such separators are disclosed in Great Britain Pat. No. 1,153,722 and Great Britain Pat. application No. 2,078,552 A. Both involve the separation of a mixture of small granular materials through a pneumatically induced fluidization of the mixture as it passes down an inclined chute. At the discharge end of each chute the mixture of materials has become somewhat stratified according to the combined density and aerodynamic properties of the component particles. Such devices have several inherent drawbacks which render them less than optimally desirable in relation to the broad range of circumstances in which agricultural separators of the dry variety are desirable.
First, separators which pneumatically fluidize the actual mixture to be separated have a limited separation effectiveness. While the upper and lower layers of the stratified flow of the mixture discharged from the end of the separator chute may be relatively pure, the layers intermediate thereto continue to comprise a mixture of particles of both densities.
This failure to achieve a distinct separation at the intermediate layers of the discharge stream is ameliorated to some extent in Great Britain Pat. application No. 2,078,552 A by horizontally narrowing the separation between the vertical walls of the chute in the vicinity of its discharge end. This has the effect of increasing the depth of the flow at the point of discharge, affording more vertical distance between the separated top and bottom layers of the mixture. Nevertheless, at some point between those two layers, the two materials of differing densities remain substantially intermixed in an interface layer. This fact precludes the achievement of optimal separation effectiveness.
A second, more profound drawback of separation methods in which the mixture to be separated is itself pneumatically fluidized arises from the fact that fluidization of a mixture is not possible if the particles of the mixture have diameters greater than or three or four millimeters. Such methods are thus effective only in separating small products such as cereal grain. Dry separation methods and apparatus which attempt to achieve separation by fluidization of the material to be separated accordingly cannot be used to sort or separate larger produce.
In order to separate large products, resort has been made to the use of fluidized beds which are constituted of a material other than the mixture to be separated. For the purpose of separating mixtures of larger solid bodies of different densities, a fluidized bed created from such a fluidization medium behaves in a manner analogous to a liquid Pieces of solid material less dense than the apparent density of the fluidized bed will float on the surface thereof. These will hereinafter be referred to as the "float fraction" of that mixture. Pieces of solid material which are more dense than the apparent density of the fluidized bed will on the other hand sink to the bottom of the bed. These will hereinafter be referred to as the "sink fraction" of the mixture. This method of separating bodies of differing densities in a mixture is aptly termed a sink-float fluidized bed separation process.
For such separation to occur, the apparent density of the fluidized bed must be intermediate the densities of the float and sink fractions of the mixture. Additionally, the particle size of the fluidization medium must be smaller by several orders of magnitude than the size of the bodies of the mixture.
The apparent density of the fluidized bed, .rho., can be expressed as: EQU .rho.=(1-.epsilon.) .rho..sub.s +.rho..sub.f
where .rho..sub.s is density of the particles of the fluidization medium, .rho..sub.f is the density of the fluidizing gas, and .epsilon. is the void fraction of the fluidized bed, a variable highly dependent upon the rate of gas flow through the bed. In fluidization, it is important to increase the rate of gas flow until bubbles appear, and the bed resembles a boiling liquid. In this condition, the bed mixes continuously and the particles thereof experience an acceptable degree of mobility.
The use of a fluidization medium other than the mixture to be separated advantageously reduces the influence on the process of other separation factors, such as aerodynamic characteristics, and reduces the process to one in which separation is accomplished substantially on the basis of differing density only. In addition, the presence of a layer of fluidization medium intermediate the float fraction of the mixture on top of the fluidized bed and the sink fraction of the mixture at the bottom thereof permits clean separation of the float and sink fractions. When the mixture itself is fluidized, an intermediate layer results which is a mixture of lighter and heavier components. By contrast, in sink-float fluidized bed separators, the layer intermediate the float and sink fractions of the mixture is composed of fluidization medium, permitting close to one hundred percent separation effectiveness.
Several types of sink-float fluidized bed separators for solid materials are described in British Pat. No. 946,480. FIG. 1 of that patent involves a fluidized bed that is reconstructed continuously on a horizontally moving conveyor. A mixture to be separated into its float and sink fractions is added to the fluidized bed and allowed to separate while the bed is transported horizontally on the conveyor. At the end of this travel, the float and sink fractions of the mixture are extracted as the fluidization material is dumped from the end of the conveyor for recycling. Such an apparatus has the drawback of being unable to contain the loss of air pressure at the edges of the bed-carrying conveyor. This results in a nonuniform density across the width of the fluidization bed, creating dysfunctional currents therewithin and impairing separation reliability at the margins of the conveyor. Poor separation efficiency results.
Other sink float fluidized bed separating methods illustrated in FIGS. 2 and 3 of British Pat. No. 946,480 involve stationary, bath-type fluidized beds. In the embodiment shown in FIG. 2, float and sink fractions of the mixture are sifted out of the fluidized bed by a rotary rake containing a plurality of banks of tines which cooperate with a grid of rods within the fluidized bed to sift low density and high density solid objects from the top and bottom respectively thereof. This particular embodiment of a separator is highly susceptible to jamming by bodies becoming lodged between the grid of rods and the moving tines of the rotary rake. Produce damage is common. Furthermore, the device must be extremely large to accommodate an effectively functioning rotary rake. Maintenance problems arising from the need to have the rake contact the bottom of the container of the fluidized bed are not uncommon. Poor separation efficiency is achieved in this design also.
The sink-float fluidized bed separator described in British Pat. No. 946,480 in relation to FIG. 3 uses a bath-type, sink-flat fluidized bed in which currents are induced through intentionally created gradients of fluidized bed density. These density gradients are a result of an uneven distribution of airholes in the bottom of the container of the bed. The purpose of the currents induced is to migrate float and sink fractions of the mixture to opposite sides of the fluidized bed, where they are removed on conveyors. Nevertheless, uneven density throughout the fluidized bed gives rise to variable separation capacity depending upon location therewithin. This lack of control of apparent density impairs the separation capacity of the device.
U.S. Pat. No. 4,322,287 pertains to a sink-float bath-type fluidized bed separator for agricultural purposes. In the device disclosed, a constant density bath of uniform depth is created from a fluidization medium in a separation chamber to which a mixture for separation is added. The float and sink fractions of the mixture are removed from the fluidized bed by perforated conveyors that pass therethrough. In such devices, product damage is common due to the use of the mechanical means employed to feed the mixture into the fluidization bed and to remove the float and sink fractions therefrom. The presence of conveyor mechanisms within the fluidized bed itself interferes with airflow therethrough, compromising the uniformity of the density of the bed and impairing separation capacity. In addition, the lack of motion of the fluidized bed itself, permits particles of the mixture to be separated to accumulate in "dead zones" in the fluidized bed which cannot be accessed by the removal conveyors. The accumulation of these particles deteriorates the quality of the fluidized bed medium, requiring careful surveillance and periodic cleaning of the bed.